The present invention relates to producing spot colors using colors in the cyan, magenta, yellow, and black ("CMYK") color gamut. It finds particular application in conjunction with producing Pantone.RTM. colors using the CMYK colorant set and will be described with particular reference thereto. It will be appreciated, however, that the invention will also find application in producing other colors from other colorant sets, and the like. For example, the method could equally well be used to produce colors using a CMYKO (cyan, magenta, yellow, orange) colorant set.
Spot colors are solid regions of a color (e.g., a Pantone.RTM. color), which are normally formulated by mixing specific amounts of various inks. Conventionally, two (2) methods have been used to produce spot colors.
The first method produces a spot color without halftoning. Instead, a special housing is mounted inside a printer to provide one extra color ink for printing. Different color inks are pre-mixed in the housing to obtain the desired color. The resulting appearance is a smooth, solid color, regardless of where the color is located in the color gamut. This method has the obvious drawback of requiring an additional housing inside the printer. Furthermore, different color inks must be mixed in exact proportions to achieve the desired color before the additional housing is filled.
The second method, on the other hand, produces a spot color using halftoning. More specifically, varying amounts of the four colorants in the CMYK colorant set are used to produce numerous sub-spots which, when viewed from a distance, form the spot color. A halftone image is made up of a plurality of halftone dots, which, in turn, are made up of numerous colored pixels. If a spot color is created by printing x.sub.1 % of cyan, x.sub.2 % of magenta, X.sub.3 % of yellow, and X.sub.4 % of black, a respective percentage of the pixels within each dot are printed for each color. U.S. Pat. No. 4,149,194, which is incorporated herein by reference, describes the basic halftoning procedure.
A halftoned image is created by comparing each of the pixels of an original, continuous tone image with the value of a corresponding location in a halftone screen. FIG. 1 shows one possible halftone screen. The halftone screen consists of a set of thresholds 2, one corresponding to each location on the page. In typical use, a halftone screen is constructed by tiling a smaller array of thresholds, known as a halftone cell, throughout the page. If the value in the image is greater than that in the screen, a value for a respective colorant is set in the halftoned image, indicating the colorant is to be applied at that location. Otherwise a value is set to indicate that no colorant is to be applied at that location. This is done separately for each of the C, M, Y and K separations, which will typically use different screens.
In a clustered dot screen, the individual cells include one or more groups of adjacent pixels having relatively low thresholds. The center of any such cluster is a dot center, and corresponds to the center of a dark area in a corresponding final image.
FIG. 2 shows an image created using the halftone screen illustrated in FIG. 1 in which the levels of 68 through 74 are selected. Note that the dark pixels are grouped into clusters of pixels, which repeat periodically. Even if the lightness of the image is not constant, the centers of the clusters follow a consistent periodic pattern.
Each halftone cell is made up of a specified number of pixels, which are arranged into rows and columns. In the example of FIG. 1, one halftone cell contains eighteen (18) sub-cells which are organized into three (3) rows and six (6) columns. Such a configuration is referred to as a 3.times.6 halftone cell.
One characteristic of the halftone screen is frequency. The frequency of the halftone screen refers to the number of halftone dots per unit of length within the screen (e.g., dots per inch). In the screen of FIG. 1, the centers of the dots have threshold `3`. These are separated by approximately 4.24 pixels (the nearest `3` to any center is 3 pixels to the side and three pixels vertically). If this screen were used with, e.g. 600 pixel per inch printing, the screen frequency would be 600 pixels/inch * 1 spot/4.24 pixels=141 spots per inch.
In addition to a frequency, a screen has associated with it an angle. The angle of the cell depends on the amount the cell is shifted from one row of cells to the next, and on the collection of thresholds within the cell. The example of FIGS. 1 and 2 shows a 45 degree screen, so called because the dominant repeat pattern occurs at a 45 (and 135) degree angle with the horizontal.
An original continuous tone ("contone") image is halftoned by individually turning on or off the dots which make up corresponding cells within the halftone screen. More specifically, the original image is scanned four separate times (e.g., one time for each of the four colors in the CMYK color model). Each scan may begin, for example, in the top, left corner of the image. During the scan, each line of the original contone image is divided into segments such that each segment represents one sample area (also referred to as a pixel). Each sample area corresponds to one cell in the halftoned image.
Four separate screens are used, one for each separation. Screens are typically designed either by hand or with computer assistance. Because an arbitrary set of four screens will not generally produce a pleasing image, it is important that the four screens be designed as a set. This is especially the case for screens with small cells, such as in the example of FIGS. 1 and 2. A collection of screens, designed to be compatible, with one screen per separation, is called a "dot set".
Through the process described above, the original contone image becomes a binary image suitable for printing, displaying and/or viewing.
Because halftone screens having a higher frequency will, by definition, have more halftone cells within a defined area, those screens result in a halftoned image having a finer resolution. The finer resolution tends to produce a spot color having a smoother, more solid appearance. The human visual system cannot resolve spatial frequencies above about 60 cycles per degree (Wandell, Foundations of Vision, Sinauer Associates, 1995, inside front cover), under the best of conditions. Under normal viewing conditions, it can only resolve 20 cycles to 30 cycles per degree. At close inspection (e.g., 6 inches), a halftone screen having a screen frequency of 240 spots per inch has a repeat pattern of 24 cycles per degree, and under most conditions appears perfectly smooth. Screens having a lower frequency, on the other hand, appear more grainy causing the halftone screen to be visible within the halftoned image. Therefore, if a spot color is produced using halftoning, it would appear desirable to incorporate high frequency halftone screens into the image.
However, there is at least one drawback to creating spot colors using a high frequency halftone screen. Specifically, the halftone screen frequency is inversely related to the number of possible color levels which can be produced. Therefore, halftone cells incorporating a high frequency halftone screen are capable of producing fewer color levels. The increments between the shades of the color levels become visibly noticeable and are referred to as contouring, when a smooth gradient is printed using a halftone cell capable of producing too small a number of levels.
For spot color printing, contouring itself is not a problem, as gradient fills are not required. However, spot color printing is commonly used to produce specific colors, such as logo colors. Therefore, if only a small number of levels is available, the closest match may differ significantly from the desired color. Such a limitation is unacceptable in applications requiring nearly exact color matches.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.